Microsoft Windows Server 2003 for Small Business Server Microsoft Exchange Mail Store unmounts

At 08:59hrs this morning I got a call from a customer who was unable to receive e-mail. Logging into their server I discovered that there were indeed messages stuck in the Local Delivery queue. I checked the Application event logs and found the following event log

Event Type: Error

Event Source: MSExchangeSA
Event Category: MAPI Session 
Event ID: 9175
Date: 8/05/2012
Time: 9:12:31 AM
User: N/A
Computer: ***DELETED***
Description:
The MAPI call 'OpenMsgStore' failed with the following error: 
The attempt to log on to the Microsoft Exchange Server computer has failed.
The MAPI provider failed.
Microsoft Exchange Server Information Store
ID no: 8004011d-0512-00000000 


For more information, click http://www.microsoft.com/contentredirect.asp.

Further investigation led to an un-mounted mail store.It was relatively easy to re-mount the store, however the support link at http://support.microsoft.com/kb/896143 leads me to think it may not be so easy all the time. After getting the service back up and running, I re-visited the logs to find that the event started at approximately 23:22hrs last night, and was preceded by the following message;


Event Type: Error
Event Source: MSExchangeSA
Event Category: Monitoring 
Event ID: 1005
Date: 7/05/2012
Time: 11:22:24 PM
User: N/A
Computer: ***DELETED***
Description:
Unexpected error <<0xc1050000 - The attempt to log on to the Microsoft Exchange Server computer has failed. The MAPI provider failed. Microsoft Exchange Server Information Store ID no: 8004011d-0512-00000000>> occurred. 


For more information, click http://www.microsoft.com/contentredirect.asp.

The support link at http://support.microsoft.com/kb/888179 did not provide much assistance in resolving the issue permanently, but I did check the allocated space and size of the mail store and the available space on disk and they were all OK.